1. Field of the Invention
The present invention relates to an image forming apparatus having charging means for charging an image carrier in contact therewith. More particularly, the present invention relates to an image forming apparatus of the type mentioned above, wherein the charging means is integrated with the image carrier so as to form a unit which can be mounted on and demounted from the stationary main part of the image forming apparatus.
2. Description of the Related Art
In general, an image forming apparatus has a charging device for charging an image carrier, such as, a photosensitive member. The charging device used in this type of image forming apparatus is a corona discharger which charges the photosensitive member by a corona discharge from discharge electrode wires under application of a high voltage to the discharge electrode wires. However, this type of charging device suffers from a disadvantage in that an image defect known as "blur" tends to occur due to denaturation of the image carrier surface by ozone produced as a result of the corona discharge, particularly when the concentration of ozone is high. The ozone generated in the charging device also reacts with nitrogen in the air to form nitrogen oxides which are deposited on the surface of the image carrier, i.e., the surface of the photosensitive member, so as to reduce electrical resistance of the surface, with the result that the charges forming an electrostatic latent image on the photosensitive member are undesirably diffused to cause an image defect known as "flow of image". In order to eliminate these image defects, it is necessary to ventilate the space in and around the charging device by supplying fresh air while drawing out ozone-containing air so as to maintain a low ozone concentration in the space around the charging device. U.S. Pat. No. 4,540,268 issued to Toyono et al. on Sept. 10, 1985 discloses a process kit mountable to an image forming apparatus. The process kit may include an ozone filter.
In recent years, there is a trend towards the personal use of copying apparatuses, laser beam printers and so forth, which has given a rise to the demand for reduction in the size and cost of these apparatuses. To cope with such a demand, a contact type charging device has been proposed in, for example, the specification of U.S. Pat. No. 4,851,960, in which an electrode with a comparatively low voltage applied thereto is made to materially contact a photosensitive member so that the photosensitive member is charged by a discharge taking place across a small gap between the electrode and the surface of the photosensitive member. The contact-type charging device have advantages, such as, small size and low voltage as compared, with corona chargers, as well as the elimination of the necessity for any charging-hysteresis erasing means, such as, pre-exposure means, thus meeting the demand for reduction in the size and price of the apparatuses. However, the contact-type charging device also generates ozone due to the discharge, so that ventilation is necessary in the region around the charging device.
FIG. 1 shows, in section, a typical known laser beam printer. This laser beam printer has a scanner unit 1 including a laser source, polygonal mirror and a correction lens system. The scanner unit emits a scanning laser beam modulated in accordance with an image signal. The laser beam is reflected by a reflection mirror 2 so as to be applied to the surface of a photosensitive drum 3. The photosensitive drum 3 has been uniformly charged by a charging device 4 which is typically a corona charger, so that an electrostatic latent image is formed on the surface of the photosensitive drum 3 as a result of application of the laser beam. The electrostatic latent image is developed by a developing device 5 so that a visible toner image is formed on the photosensitive drum. The toner image is then transferred to a transfer material 7 by means of a transfer charger 6 and is fixed to the surface of the transfer material 7 by a fixing device 8. Any residual toner remaining on the photosensitive drum 3 is removed by a cleaning device 9.
In this printer, the photosensitive drum 3, the charger 4, the developing device 5 and the cleaning device 9 are supported integrally so as to form an image forming unit K which is detachably and replaceably mounted on the stationary main part of the printer.
In operation of the printer, a fan 11 creates a flow of air which is introduced from the main part of the printer into the image forming unit K through a beam exposure opening 12 as indicated by an arrow A. The air is discharged through a discharge window 13 which is provided in a region between the charger 4 and the cleaning device 9 and extending in the longitudinal direction of the photosensitive drum 3, whereby ozone which is generated in the region around the charging device 4 is conveyed out of the printer.
However, the following problem is encountered when a contact-type charging device is used in place of the corona charger as the charging device 4. Namely, in such a case, air tends to stagnate near to the portion of the surface of the photosensitive drum 3 opposing the charger 4. In particular, the velocity of the air flow is extremely low at regions near both ends of the photosensitive drum 3 which closely oppose the inner surface of the unit K. Therefore, ozone can hardly be removed from the regions around the portion of the photosensitive member 3 opposing the charging device 4 particularly at both axial ends of the photosensitive drum. As a result, image defects such as blur or flow of image tend to occur at the portions near both axial ends of the photosensitive member.
In order to realize an efficient discharge of air from the region around the portion of the photosensitive drum 3 opposing the charging device 4, U.S. Pat. Application Ser. No. 497,889, filed Mar. 23, 1990, proposes an arrangement in which an air discharge window is formed in the wall of the unit K adjacent the charging device 4 and an air discharge duct is connected to the main part of the printer opposing the discharge window, so as to form a flow of air in the longitudinal direction of the photosensitive drum 3. This arrangement, however, undesirably increases the size and raises the cost of the printer due to provision of the air discharge duct. In addition, the mechanical strength of the unit K is reduced due to the provision of the discharge window.